1. Field of the Invention
The present invention is directed to deposited thin films of semiconductors and dielectrics. The present invention further relates to the use of these thin films in detection, analytical, contact, and biomedical applications. Applications of these thin films include desorption-ionization mass spectroscopy, electrical contacts for organic thin films and molecules, optical coupling of light energy for analysis, biological materials manipulation, chromatographic separation, head space adsorbance media, media for atomic molecular adsorbance or attachment, and substrates for cell attachment.
2. Description of Related Art
There is a great deal of interest in semiconductor and semiconductor-based (e.g., oxides, nitrides) materials with large surface to volume ratios; i.e., with large surface area. The reasons for this are two-fold. First, because of the large surface area, such materials are open to widespread surface chemical attack and, therefore, can be used as separation or release layers. These are needed in a variety of applications including MEMS (microelectro-mechanical devices), interconnect dielectric, micro-sensor, micro-fluidic and wafer separation applications. Secondly, these materials can be used as cell and molecule attachment layers, contacts and sensor materials. In addition, such materials can be very compatible with microelectronics. There are various approaches to producing large surface to volume (i.e., large surface area) materials. The technique attracting the most attention today is based on electrochemical etching. When electrochemical etching is used to produce large surface area silicon, the resulting material is commonly termed porous silicon. Porous Si was first obtained in 1956 electrochemically by Uhlir [A. Uhlir, Bell Syst. Tech. J. 35, 333 (1956).] at Bell Labs but it was not until 1970 that the porous nature of the electrochemically etched Si was realized [Y. Watanabe and T. Sakai, Rev. Electron. Commun. Labs. 19, 899 (1971). Recent discussions can be found in R. C. Anderson, R. C. Muller, and C. W. Tobias, Journal of Microelectro-mechanical System, vol. 3, (1994), 10.
The starting material for this wet etched conventional porous Si material is either conventional silicon wafers or thin film Si produced by some deposition process such as low pressure chemical vapor deposition (LPCVD) or plasma enhanced chemical vapor deposition (PECVD). In the electrochemical wet etching process the sample is exposed to a wet solution and a current is passed through a contact to the etching sample, through the etching sample, through the solution (e.g., a mixture of hydrofluoric acid, water and ethanol), and through an electrode contacting the solution (the cathode; e.g., platinum). This current causes the xe2x80x9cpittingxe2x80x9d or etching of the Si producing a porous network structure.
In the electrochemical (anodic) etching process the structure (e.g., pore size and spacing) and the porous-Si layer thickness are controllable by the resistivity of the silicon itself (magnitude and type), current density, applied potential, electrolyte composition, application of light, temperature, and exposure time. For sufficiently long exposures and for sufficiently thick starting material, this electrochemical etching process can be continued to the point where nanoscale structure (i.e., features of the order of nanometers) is obtained. The silicon features are a continuous single crystal when the sample is etched from a single crystal wafer, as is usually done, or polycrystalline silicon when the sample is etched from a deposited film. All these conventional (electrochemically etched) porous silicon materials are distinguished by (1) being the result of a wet, electrochemical etching process, (2) requiring a contact on the sample during this wet etching, (3) having generally disconnected pore regions which can be connected after extensive etching, and (4) being the result of a sequential processing first necessitating formation of the silicon and then necessitating subsequent wet etching. Besides the complexity of having to prepare, use, and dispose of wet chemical etching baths, these wet etched porous materials suffer from a problem of residual etching species and products remaining in the pores.
An alternative approach to producing a porous silicon thin film was shown by Messier (R. Messier, S. V. Krishnaswamy, L. R. Gilbert, and P. Swab, J. Appl. Phys. 51, 1611 (1980).). In this approach, a film with a spatially varying density was deposited. This film was subsequently wet etched, which removed at least some of the low-density region. As a result of this wet etch step, there was an increase in the film surface to volume ratio.
Intense research activity in porous semiconductors has been stimulated over the last decade by the discovery of room temperature visible light emission from electrochemically prepared porous Si in 1990 by Canham (L. T. Canham, Appl. Phys. Lett. 57, 1046 (1990)). Soon after Canham""s discovery, further intriguing properties of electrochemically prepared porous silicon were also realized, such as gas sensitivity, bio-compatibility and ease of micromachining, etc. (I. Schecter et al., Anal. Chem. 67, 3727 (1995); J. Wei et al. Nature 399, 243 (1999); L. T. Canham et al., Thin Sold Films, 297, 304 (1997); P. Steiner et al., Thin Solid Films 255, 52 (1995)). All these demonstrated applications to date have been based on the porous silicon material produced by electrochemically etching a wafer or deposited film of silicon.
The approach to producing a high surface area to volume ratio material in the present invention is to use deposition to grow as-deposited high surface-area films. In fact, we show that, with careful control of deposition parameters and techniques, we can attain a spectrum of films with tunable, surface area to volume ratios. This tunability allows morphology from continuous (surface area is the film area; i.e., void free) films to materials with up to about 90% porosity. We show such films have tunable chemical and physical properties such as variable species adsorption, light reflectance, and light absorption properties. Depending on the deposition technique and parameters, these thin films may be continuous (void free) or may have voids between columns and clusters. The present approach uses deposition performed at low temperature and tailored to attain the required morphology. There is no specific etching step involved and no wet processing. The present inventors have demonstrated that the present invention can be used to control void size and void fraction, that a columnar/void network morphology can be produced and that the columns can be polycrystalline material. In the demonstrations provided here for these controlled morphology films, plasma enhanced chemical vapor deposition (PECVD) is used to give continuous (void free) films, physical vapor deposition (PVD) is used to give an intermediate morphology, and PECVD is used to produce high void density (high surface area) material. Due to the deposition approach, high porosity (of up to approximately 90%) is attainable in the high void density material without any specific etching step. None of the controlled morphology films of this invention requires contacts, wet processing, or both. Also unique to the present invention is its ability to fabricate these deposited films, with designed morphology matched to the application, on various types of substrates including glass, metal foils, insulators, plastic, and semiconductor-containing materials including substrates with circuit structures.
As noted, the high void density morphology material is demonstrated using PECVD. In particular a this columnar/void network silicon was demonstrated by use of a high density plasma tool (e.g., Electron Cyclotron Resonance Plasma Enhanced Chemical Vapor Deposition (ECR-PECVD) tool (PlasmaTherm SLR-770)) using hydrogen diluted silane (H2:SiH4) as the precursor gas at substrate deposition temperatures less than or equal to about 250xc2x0 C. This tool plays off silicon etching and deposition to create a two-dimensional silicon array and analysis has demonstrated that silicon column size is controllable and the spacing between columns is controllable. The resulting columnar/void network structure is nanoscale in feature size and fully developed after a film thickness in the range of 10-20 nm is established. This enables the direct deposition of high porosity crystalline or amorphous silicon on any substrate and at any thickness greater than about 10 nm. The columnar/void semiconductor films produced by the present invention may be converted to insulators or metallic compounds through in situ or ex situ processing. In addition, other layers such as anti-reflective (AR) coatings or functionalizing layers may be deposited after or before deposition of the columnar/void network material. By varying the deposition parameters in a high-density plasma tool, either continuous (void free) intermediate, or high void density material may be produced.
As noted, the prior art contains two approaches to creating porous silicon: (1) wet electrochemical etching of deposited silicon or of silicon wafers to produce a porous silicon with a xe2x80x9ccoral-likexe2x80x9d morphology of polycrystalline or single-crystal silicon xe2x80x9cfingersxe2x80x9d or (2) deposition to produce a material of varying density amorphous silicon followed by wet etching. The former material is the subject of a great deal of research and development activity. However, it requires wet chemical etching for its formation. The latter material suffers from only being demonstrated in the amorphous phase, from having a morphology that varies with thickness, and from requiring wet etching to control void density. Since its xe2x80x9cvoidsxe2x80x9d are believed to be lower material density regions, as opposed to true voids, it requires this subsequent wet etching for true void tailoring and control. The high surface area to volume ratio silicon of this invention requires no wet processing due to its nanoscale features and voids. It has a fully controllable morphology and porosity and can be in the polycrystalline or amorphous phase as desired.
This invention is the creation of controllable and tailorable surface area films from continuous films (no voids) to high surface area to volume ratio films (high void density) by deposition at low temperatures. The film morphology (surface area to volume ratio) is tailored to the film use. These materials are particularly suitable for deposition on glass, plastic or substrates requiring low processing temperatures such as substrates containing previously formed sensor, electronic or opto-electronic devices and circuits. Due to the wide, demonstrated void volume range possible for the materials of this invention, they can be used for a number of applications including sensing, airgaps (optical mixing, microfluidics, molecular sorting, low dielectric constant structures, etc.), fixing and electrically contacting molecules and cells, and molecular desorption applications.
The present invention is directed to deposited film structures having morphologies that are variable and tailorable from a continuous film (no voids) to a film comprising: (a) a network of columnar-like units in a continuous void; and (b) a substrate to which the network of columnar-like units is adhered. These films are based on chemical elements such as silicon, germanium, carbon, hydrogen or mixtures thereof. In a preferred embodiment, the substrate supporting these films is composed of a material such as glass, metal, ceramic, insulation material, plastic material, silicon or semiconductor-containing material. This invention covers the use of deposited AR films on these deposited films for enhancement of light coupling. Table 1 summarizes the deposited variable morphology films of this invention and some examples of morphology-applications tailoring.
The films of this invention are deposited at low temperatures and are morphologically tailored for specific applications, examples of which are noted in Table 1. In an embodiment of this invention the material is a continuous semiconductor film having no voids and deposited by PECVD. In an embodiment of this invention the material has an intermediate morphology with a low void density and is deposited by PVD. In an embodiment of the invention, the films are a nanostructured columnar/void material with a network of units collected in clusters and formed by deposition via a high-density plasma. In this latter case, the spacing and height of the network of columnar-like units are adjustable by variables including oxidation, silicidation, etching, voltage, current, voltage between plasma and substrate, substrate temperature, plasma power, process pressure, electromagnetic field in the vicinity of the substrate, deposition gases and flow rates, chamber conditioning, and substrate surface. Furthermore, by using the latter methodology and modifying the deposition conditions, not only can the nanostructured columnar/void material be deposited, but also the mentioned continuous and low void density films can be produced.
The present invention is also directed to a method for detection of analytes in a sample. The method for the analysis of a sample comprises (a) applying the sample to a deposited thin-film; and (b) analyzing the sample by a detection means. More particularly, the method comprises: (a) selecting one of the film morphologies of Table 1, as dictated by the specific application, (b) applying the analyte onto the selected film structure described above; (c) transferring the sample into a detection device; (d) discharging light such as laser energy on the sample, thereby transforming the analytes in the sample into charged particles which detach from the film structure enter a vacuum having an electric field and move through the detection device or detection means to a detector. The sample to be selected, includes organic chemical compositions, inorganic chemical compositions, biochemical compositions, cells, micro-organisms, peptides, polypeptides, proteins, lipids, carbohydrates, nucleic acids, or mixtures thereof. The peptide, polypeptide, or protein sample has a molecular weight greater than 0 Daltons. The sample is directly applied to the film as a liquid and thereafter evaporated to dryness. And, the sample is in aqueous or organic solution or suspension.
The criteria for selecting a particular film is based on properties of the film such as laser-light reflection, optical absorption, species absorption, and ambient absorption. The selection of deposited thin-films which are used for the above method includes continuous film, a column structure film, a columnar-void film, and a mixture thereof. The film is essentially a single homogenous film or a heterogeneous mixture of more than one film. The heterogeneous mixture is a patterned columnar void network of films.
The analyte may be applied by an application/removal protocol for such application, which is also the subject of this invention. Application of sample to a film is either by (1) adsorption from a solid, liquid or gas; or (2) direct application to the surface of the deposited thin film as a solid or liquid. In an embodiment of the invention, the sample is applied to the film directly from a chemical separation means including liquid chromatography, gas chromatography, and deposited thin-film chromatography.
In one embodiment of the invention, the detection means or detection device for the above method includes light desorption mass spectroscopy, antigen-antibody reaction detection, fluorescence detection means, optical detection means, radioactivity detection means, electrical detection means, chemical detection means, antigen-antibody reaction detection and combinations thereof. The chemical detection means involves dye or coloring means and colorimetry or visualization. Preferably, the detection device uses time of flight analysis for species identification. These films and the morphology selection and tailoring outlined above may also be used for a means of chemical separation such as chromatography.
The film morphology selection is based on the properties needed for the application. For example, in applications where sample confinement is an issue, the spacing and height of the network of columnar-like units of the columnar/void network morphology film structure may be adjusted to reduce lateral drop spreading of the analyte. The film structure is selected from Table 1 as needed, based on one or more film attributes: low laser-light reflection (which may also include the use of AR coatings), strong optical absorption, species adsorption, and ambient adsorption. The method for separation of analytes in a sample using a chemical separation means comprising a deposited thin film, the method comprising the steps of: (a) applying the sample to the deposited thin film; (b) passing the sample through the deposited thin film; whereby the analytes of the sample migrate through the deposited thin film thereby separating each analyte in the sample by mobility of each analyte. Forces for the passing of the sample in step (b) include gravity, centrifugal force, electric field, and pressure gradient. More particularly, the present invention is directed to a method for separation of analytes in a sample comprising: (a) exposing the analyte to the film structure as described above; and (b) moving the sample through the film structure whereby the analytes of the sample migrate through the network of columnar-like units of the film structure thereby separating each analyte in the sample by properties such as the mobility of each analyte. The mobility of each analyte is dependent upon mass, charge to mass ratio, physical interaction, size, or shape. The spacing and height of the network of columnar-like units of the film structure is adjusted to control the migration of targeted analytes.
The present invention is also directed to a method for selective adherence of analytes in a sample comprising the steps of: (a) modifying, functionalizing or patterning in a physical or chemical manner the deposited thin-film; and (b) applying the sample to a deposited thin-film, whereby a particular analyte or analytes from the sample adhere to the deposited thin-film. More particularly, the present invention is directed to a method for selective adherence of the analytes of a sample comprising: (a) modifying the film structure described above such that specific regions have been physically shaped or chemically functionalized to capture analyte; and (b) exposing the sample to be analyzed to the film structure whereby particular analytes in the sample adhere onto the film structure in pre-specified regions. The analyte-containing sample may be in solid, gaseous or liquid form. A surface of the film structure of step (a) may be functionalized with a molecule or molecules including: reactive, non-reactive, organic, organo-metallic and non-organic species, thereby allowing the surface to be specified for reaction with particular analytes. The surface of the film may be physically defined; for example, a hole, receptacle, or confining pattern created by a subsequent functionalization, surface treatment, molecular attachment or film deposition may be defined to segregate analyte to specific regions of the film. Chemical modification to the film structure may comprise steps such as oxidation, reduction, addition of a chemical element, hydophobicity or hydrophylicity treatments, lipid attachment, Lewis acid mediated hydrosilylation, or silicidation. In one embodiment of the invention, the film is patterned by lithography of the film or of a subsequently positioned material. In another embodiment of the invention, the film is modified to adhere an antibody, antibodies or other chemical moiety, with the sample. A detection means is then used to detect antigen-antibody reaction or the adherence of the antibody, antibodies or other chemical to the film. In a further embodiment of the invention, the film is modified to adhere cells including neuronal, glia, osteoblasts, osteoclasts, chondrocytes, kerotinocytes, melanocytes, and epidermal cells; whereby the cells proliferate on the film. In a further embodiment of the invention, the film is modified to adhere cells including neuronal, glia, osteoblasts, osteoclasts, chondrocytes, kerotinocytes, melanocytes, and epidermal cells; whereby the cells proliferate on the film. The film can be modified so that cell proliferation is controlled or restricted. Also, the film with cells adhered can be placed in vivo.
The present invention is also directed to a method for determining a property of a particular analyte in a sample comprising: (a) modifying a first film structure; (b) modifying a second film structure; (c) applying a sample to the first and second film structures; and (d) analyzing the first and second film structures to determine which film structure interacted with the particular analyte in the sample. It is one embodiment of the invention, whereby the first and second film structures are separately modified by various treatments such as attachments or Lewis acid mediated reactions on the surface of the first and second film structures.
The present invention is directed to a method for specifying a particular reaction comprising the steps of: (a) functionalizing the film; (b) applying a sample with multiple analytes to the functionalized film; wherein a particular analyte in the sample reacts in the presence of the functionalized film. The chemical property of a particular analyte can be determined using the above described method. In one embodiment of the invention, the first molecule adheres to the film structure in a specified orientation. In another embodiment of the invention, the second molecule is selected from the group including: nucleic acids, proteins, organic and organo-metallic reagents. In a further embodiment of the invention, the first molecule adheres to the film structure in a specified orientation and the second molecule reacts with the first molecule.
The present invention is directed to a method for screening a library of samples comprising the steps of: (a) applying each sample in the library of samples to a deposited thin film; and (b) analyzing each sample by a detection means. In one embodiment of the present invention is further directed to a method for screening a library of compounds to identify a particular characteristic in the compounds comprises: (a) modifying a film structure as described above; (b) applying a compound to the film structure; (c) analyzing the film structure with the compound; and (d) comparing analysis of the film structure with compound to the film structure without the compound so as to determine whether a reaction has taken place.
The present invention is directed to a method for promotion of cell analysis, cell products, and/or cell growth comprising: (a) modifying the surface and structure of the film structure as described above; and (b) exposing a sample to the film structure; whereby particular cells in the sample adhere onto the film structure and wherein the particular cells proliferate on the film structure. In one embodiment of the invention, the cells are selected from the group including: neuronal, glia, osteoblasts, osteoclasts, chondrocytes, kerotinocytes, melanocytes, and epidermal cells. The film structure may be modified in step (a) so that the cell growth is controlled or restricted. The sample (film structure with cells) may be placed in vivo.
Using these films for molecular attachment may also be explored for producing contacts to organic semiconductors and molecules used in molecular electronics. Such films can be excellent contacts due to the carrier injection capabilities of semiconductors or due to their use as silicides with their specific potential for high conductivities.